1. Field of Invention
Wood burning has been a practice since the old ages but is still a primitive process, which can be improved with respect to the completeness of the combustion and the emissions. The difficulties are caused by the small size of the fireplace, the individuality of the wood logs and the charge procedure with intermittent feed of new fuel from time to time. Many inventors have worked with these problems and have reported solutions, which are said to be a satisfactory answer to these problems. Nobody has, however, yet presented a universal solution which can handle all different situations in the practice with different fuels, different routines for combustion etc. Several cooperating technical measures are required to achieve this goal.
2. Description of Related Art
It is thus necessary to rely on several means for efficient and environmentally acceptable combustion of wood logs considering the variations with respect to the activity, moisture content, surface/volume ratio, etc in a collection of wood logs. The present invention is concerned with a new burner for wood which gives a very good combustion result during the different conditions of the practice in spite of a very simple mechanical design.
The burner, which will be called the basket burner in the following, can be put up in an open fireplace but it can also be built in as an integrated part of wood stoves of different kinds, inserts, tile-stoves, etc. The burner is also suitable for combustion of thicker wood logs, bundles of thin wood logs etc.
A primary purpose with the invention is to produce a useful configuration of the charge of wood logs for good process technical conditions with respect to the interaction between the wood logs in the fuel charge, the combustion air and the combustion gases. This will give a very efficient combustion process with small emissions of harmful hydrocarbons and carbon monoxide.
A second purpose is to produce conditions for a very complete combustion of the fuel wood with a small residue of unburnt material.
A third purpose is to produce conditions for a convenient and fast lighting of the wood charge with small emissions also during this part of the operation, which is critical from an emission point of view.
A fourth purpose is to reduce emissions when a new charge of wood logs is put on the fire.
The burner according to the invention meets the above desiderata in a very efficient and surprisingly simple way. The invention is a result of a new insight into the problems which arise during combustion of wood.
There is a very complicated interplay between the wood logs in a burning fire. This interplay contains both positive and negative effects. The successive lighting up when an already burning and glowing log of wood puts fire to adjacent wood logs is a well known example of positive interplay. Negative effects are shielding from air supply or that there will be no fire only smoke, e.g. caused by the pyrolysis gas pushing away the combustion air.
The supporting interplay between the wood logs in a burning fire is diminished towards the end of firing. The ash layer contains then unburnt charred ends of the wood logs. A few charred wood logs remain standing up against the wall of the fireplace as a monument over unsatisfactory combustion technology.
The different wood logs in a burning fire thus serve as a kind of support burners for each other. These circumstances have not been taken sufficiently seriously earlier.
The combustion chamber, the means for supply of primary and secondary combustion air and the grate are frequently designed without due consideration to the conditions which have been touched upon above. The wood is frequently put directly on a plane grate, see e.g. the description of modern wood combustion techniques for so-called "clean burners" in "Wood'n Energy", December 1984, pp 14-18.
The combustion result is frequently very good in the laboratory with these "clean burners", particularly if the combustion is supported with a catalyst insert. In practice when the fuel charge contains wood logs with different size and reactivity, the result may nevertheless be not so good with unburnt wood residues in the ash layer. The reason for this is that the internal burning support within the fire has been insufficient.
A primary requirement on efficient burning means should be that they shall produce an efficient fire support during the whole burning process particularly in the beginning and at the end of the burning process, when new fuel is put on the fire.
The basket burner constitutes a highly efficient but at the same time surprisingly simple solution of the fire support problem which has been discussed above. The combustion becomes complete and the emissions to the surroundings of the stack gases are small.
The basket burner also exhibits several other important advantages. The lighting is very convenient and rapid. It is not necessary to use paper, birch-bark, fire sticks etc. in the fuel charge. The fire is lighted directly by means of a candle, cigarette lighter etc. Additional charges of fresh fuel is lighted up very rapidly. The fire then takes care of itself.
The delivery of radiation heat from the fire to the surroundings is very good, particularly during the glow phase. The basket burner then exposes a large glowing surface, which is radiating towards the space outside instead of towards the upper parts of the fireplace.
Another advantage is that the basket burner permits a large charge of fuel in a given combustion chamber. This is of value for fireplace inserts, which have to fit into an available limited space. The ash tray can be eliminated thanks to the complete combustion. The basket burner itself keeps the ash.
The most important thing is, however, that the basket burner produces excellent combustion technical and combustion chemical conditions. The fuel charge is lighted completely and is then burning steadily towards the glowing phase or to the instant when a new fuel charge is to be put into the burner.